Polyolefin films, especially polyethylene films, are frequently formed as blown films. Film blowing involves continuously extruding a polymer melt through a die in order to form a continuous cylinder of viscous polymer, and then expanding the diameter of the cylinder through, for example, a pressure differential between the inside and outside of the cylinders.
Bags may be formed by collapsing the polymer cylinder along its diameter perpendicular to the extruding (machine) direction in order to provide two parallel layers of polyethylene film, the two layers being contiguous along two parallel edges separated by a width of approximately II times the radius of the cylinder. The bag's sealed end is formed by heating the layers sufficiently to form a small region of polymer interdiffusion between the layers along a line traversing the width of the bag from edge to edge. Such a line is generally perpendicular to the extruding direction and may be formed by narrow, heated, metal bars that contact the outer surfaces of the layers.
The film may also be formed into a continuous sheet. When the cylinder is at a desired radius approximately equal to II times the reciprocal of the width of a take-up spool and the cylinder's wall thickness is approximately equal to the desired film thickness, the film is parted longitudinally and then wound onto the take-up spool.
A critical property in polymer packaging, especially in the formation of bags, applications is known as "hot tack". Hot tack is the strength of a heat seal immediately after sealing and before the seal has cooled down and reached maximum strength. Bags formed from polymer film having improved hot tack are less prone to fail during and after processing along the region of polymer interdiffusion that forms the closed end of the bag. In one hot tack determination method, the automatic heat sealing packaging process is simulated by forming heat seals and testing them in a short period of time, i.e. less than one second.
Hot tack determines the maximum packaging speed. See for example, U.S. Pat. No. 3,478,131. It is believed that both the melting and crystallization of a polymer behavior strongly affects its hot tack properties. Materials having desirable hot tack display a low melting point and a broad melting endotherm in a differential scanning calorimeter, allowing heat seals to form at the lowest possible temperature. It is also desirable that the material solidifies, or crystallizes at a temperature as possible so that the heat seals develop strength quickly.
Blown films prepared from conventional Ziegler-Natta catalyzed polyethylene suffers from relatively poor hot tack compared to metallocene catalyzed polyethylene. This difference is believed to result from the metallocene based material's broader melting endotherm. However, further hot tack improvement would be desirable even in metallocene-catalyzed polyethylene.